The present invention relates to a rotation control system for a disk player which reproduces disks of different sizes, and more particularly to a rotation control system which operates during starting of a disk motor.
In reproducing signals from an optical disk such as a digital audio disk or a video disk, it first is necessary to focus a laser beam on the disk surface by adjusting the position of the objective lens of an optical pickup.
In adjusting the focus, a method is employed in which the objective lens is oscillated in the perpendicular direction with respect to the disk by means of an output signal (1-2 Hz) of an oscillation circuit (referred to as a focus search circuit), while keeping the objective lens of the optical pickup a certain distance away from the disk. A focus servo loop is turned on immediately upon detection of an error signal with approach of the focal depth of the objective lens to the reflection surface of the disk, and the objective lens is drawn within the focal depth and the focus search circuit turned off simultaneously.
In this case focus adjustment may be carried out in a condition where the disk is kept stationary. However, sometimes focus drawing is not feasible because of pits that are found in the vicinity of a position to which the beam of light is pointed. In particular, in the case of a disk player in a vehicle, there is an inconvenience that focus adjustment is difficult due to the influence of vibrations of the vehicle. For this reason, a stabilized focus drawing will become realizable if the influence of the pits at the position where the beam of light is pointed is averaged out by adjusting the focus in a condition in which the disk is kept rotating.
Now, currently, there are available two kinds of digital audio disks; a large-sized disk with an outer diameter of 12 cm and maximum reproduction time of about 75 minutes; and a small-sized disk with an outer diameter of 8 cm and maximum reproduction time of about 20 minutes. Since the optical pickup reads both kinds of disk the same way, there have been developed disk players available for reproducing disks of different sizes.
With such disk players, there has arisen an inconvenience in that the time taken by the disk motor to attain a prescribed rotational frequency, for example, the rotational frequency at which the influence of the pits is averaged out (about 20 rpm), varies with the size of the disk, since the weight of the disk varies with the size of the disk, assuming that the starting torque of the disk motor remains constant. In particular, in the case of installing a disk motor within a limited housing space such as in the case of a disk player for vehicles, the tendency becomes more apparent because it is not possible to use a motor with a performance margin.